import java.util.Scanner;

public class MC0316众符 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        char[] ch = scanner.next().toCharArray();
        int[] arr = new int[26];
        int max = arr[0];
        char modeChar = ' ';
        for (int i = 0; i < ch.length; i++) {
            arr[ch[i] - 97]++;
        }
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] > max){
                max = arr[i];
                modeChar = (char) (i + 97);
            }
        }
        System.out.println(modeChar);
        scanner.close();
    }
}
